Ecological study of Carassotrema schistorchis in wild silver carp, Hypophthalmichthys molitrix

Carassotrema schistorchis is a digenea parasite whose juvenile and adult stages parasitize the intestinal tract of definitive host fish. There is no available ecological information about this parasitic infection in wild host fish. The present work aimed to investigate the infection rate and distribution of C. schistorchis in wild silver carp, Hypophthalmichthys molitrix, and to assess the influence of environmental factors on infection by the parasite. From October 2016 to May 2019, a total of 554 wild silver carp were collected from two reservoirs. These collected fish were weighed, measured, and dissected to observe the presence of C. schistorchis following standard methodology. Ecological analysis results showed that the frequency distribution of C. schistorchis in the host population was aggregated distribution. The abundance of C. schistorchis exhibited a positive correlation with the host age and condition factor of silver carp, but it showed no relationship with water temperature, pH or DO. The condition factor value of infected fish was significantly higher than that of uninfected fish. These results indicated that food intake, not environmental factors, directly determined infection by the parasite in the host, indicating that an increased food intake would increase the risk of infection. Thus, the number of parasites could be used as an indirect biological marker for assessing the effectiveness of silver carp in filter-feeding on plankton. The following analysis indicated that silver carp with a body length of 50–55 cm showed the highest efficiency in controlling phytoplankton biomass. This study provides important information for understanding the ecology of C. schistorchis in wild silver carp.